Varenicline tartrate salt, 7,8,9,10-tetrahydro-6,10-methano-6H-pyrazino[2,3-h][3]benzazepine, (2R,3R)-2,3-dihydroxybutanedioate (1:1), has a molecular weight of 361.35 Daltons and is described according to the following formula:

Varenicline tartrate is marketed by Pfizer under the trade name of CHANTIX™ as a partial agonist selective for certain subtypes of nicotinic receptors and indicated for smoking cessation.
Varenicline base and a variety of salts thereof are disclosed in U.S. Pat. No. 6,410,550, EP 1044189, and EP 1659114.
Crystalline forms of Varenicline citrate and succinate salts are described in the U.S. Pat. Nos. 6,787,549 and 6,794,388, respectively.
Varenicline L-tartrate and its crystalline forms A, B, and C are described in the U.S. Pat. Nos. 6,890,927 and 7,265,119.